1. Field of the invention
The subject invention concerns a dewatering felt to be used as a wet press felt in the press section of a papermaking machine.
2. Brief Description of the Prior Art
A papermaking machine comprises three different sections. In the forming section the stock suspension fed onto traveling forming fabric or between two such fabrics. The majority of the water is removed from the stock, so that a continuous paper sheet is formed on the fabric. The formed sheet is carried into the press section, where some more water is removed by pressing. Finally, the sheet is dried in the dryer section by being pressed against hot cylinders, so that the moisture in the paper sheet is vaporized.
An important part of the papermaking process is dewatering efficiency in the press section. It is much more economical to remove the water in the press section than to vaporize it in the dryer section. The energy consumption is considerably higher in the dryer section than in the press section.
In the press section of the papermaking machine the formed sheet is pressed to a higher dry content through repeated pressings, usually in roll press nips. The sheet is carried throgh the press nip together with one or several endless textile fabrics, that are generally referred to as press felts or wet felts.
The press felt usually comprises a soft surface layer closest to the paper web, which said surface layer is compressed to a rigidity without any air volume. Under the surface layer is usually arranged a base fabric, which is designed to retain most of its void volume, even when a maximum pressure is applied on the press felt.
The purpose of this design is that this part of the felt is supposed to absorb an optimal amount of water from the paper web at the compression of the web and the felt in the press nip and after this retain as much as possible of removed water, that later is released in a suitable manner, before the felt is reentered into the press nip.
In a nowadays common type of roller press the bottom press roller is formed with cavities in the form of suction holes, on the inside connected to a vacuum source, or lengthwise extending grooves (known as Venta or grooved roll) or blind drilled holes. The cavities in such a roll completely or partly replace the base part of the felt or supplement this as a water-absorbing medium, when the paper sheet and felt are compressed in the press nip. Normally, grooved and blind-drilled press rolls are used at the end of the press section at high linear pressures and high speeds.
When the paper sheet together with one or several press felts is carried into the press nip, the water from the fiber web is forced into the felt and then together with the amount of air stored in the surface layer of the felt it is forced backwards into the void volume of the base fabric and/or into the void volume of the press roll. Some water is also allowed to flow forwards or backwards in the lengthwise direction inside the felt. The relationship between these flow directions depends e.g. on the speed of the machine and on the design of the felt and its ability to handle the water removed from the sheet.
Several theories have been put forward about what is going on in the paper sheet and felt during the press process itself. The exerted nip pressure is the same for both paper sheet and felt, while on the other hand the hydrodynamic pressure is considerably higher in the sheet than in the felt. This pressure difference provides the driving force for the transportation of the water from the sheet to the felt.
The minimum thickness of the sheet and that felt probably appear at the same time and somewhat after mid nip. The sheet is considered to reach its maximum dry content at the very same moment. After that, the expansion is beginning in the sheet as well as in the felt. During this expansion a vacuum is created in the paper sheet and in the surface layer of the felt, both of which have been totally compressed during the maximum pressure. Available water is flowing back from the inside and base layers of the felt to the surface layer of the felt and further into the sheet to re-establish the pressure balance. This phase provides the driving force of the re-wetting inside the press nip.
In the prior-art felt constructions it is common practice to form the felt with a considerably denser surface layer facing the paper web than the backside structure and it has not been unusual to use lengthwise extending fibers on the web facing side. The high capillary forces together with the largest vacuum of the felt structure during the expansion phase have been absorbing water from an open backside structure toward the surface layer, whereby the vacuum rapidly decreases in the surface layer. When the vacuum of the sheet thus rises considerably and the flow resistance in the contact face of felt against the sheet decreases this results in high re-wetting and low paper dry content.
The purpose of this invention is to create and above all to maintain a vacuum pressure which is as high as possible in the surface layer of the felt during the expansion phase by counter-acting the water-flow from the interior of the felt to the side facing the paper web.